1. Field of the Invention
The present invention relates to sheet-like polyethylene terephthalate materials having little surface roughness, a process for their preparation by successive exposure of the surfaces to two polarized UV beams produced by the decomposition of excimers, and the use of polyethylene terephthalate films treated in this manner as substrate material for thin ferromagnetic metal layers.
2. Description of Related Art
For many applications of sheet-like polyethylene terephthalate material, it is necessary to provide this material with a defined surface roughness. The roughnesses generally result in an improvement in the adhesion, for example in the case of adhesive bonds, the printability or the wetability. A roughened surface is also advantageous for stacking of this material or for winding of corresponding film webs. Polyethylene terephthalate (PET films) having a defined roughness are very important especially for the production of magnetic recording media which consist of a polymeric substrate, a coherent, thin ferromagnetic metal layer applied to the surface of the substrate by a PVD (physical vapor deposition) method and, if required, a protective layer formed on the metal layer. The surface roughness of the films ensures little wear of the magnetic layer under tribological stress due to the head. Roughening of the polymer surface also improves the adhesion between the polymer and the magnetic layer. In addition, the mechanical and chemical stability of the metal layer give rise to particular requirements with regard to the procedure. However, these problems cannot be viewed in isolation since any type of optimization of the mechanical properties of the magnetic recording media under discussion must never lead to deterioration in the magnetic properties, ie. the properties relating to information storage.
The reduction in the wear of the magnetic layer due to the tribological stress caused by the head is achieved, inter alia, by avoiding, by means of the surface roughness, direct contact between the head and the metal layer over its entire, macroscopic contact area. In order to achieve this effect, it is important for the mean spacing between the projections on the surface to be small compared with the dimensions of the macroscopic contact area of the head. The depth of the surface roughness is limited by the fact that, if said roughness is too great, the magnetic recording and playback properties of the applied layers deteriorate as a result of the signal-to-noise ratio.
In order to achieve the desired properties of the polymer films for use as a substrate of magnetic thin-layer media, films in which a defined surface roughness is produced by incorporating and applying very small particles of an inert material have been used to date (eg. EP 0 153 853). Although these films are suitable, the production of such films requires particular and expensive production methods.
The other conventional processes for the treatment of polymer surfaces in plasma, by glow discharge, corona discharge, flame application, chemical etching or ion bombardment prior to metallization can never be completely satisfactory. The insufficient controlability of the energy effect and/or residual gas control and the resulting contamination by decomposition products were particularly significant here.
The previously described processes for the UV exposure of polymer surfaces to continuous UV lamps, for example mercury vapor lamps, to improve the coatability, to improve the printability of polyolefins (US-A Pat No. 4,933,123), or to increase the adhesive strength of adhesives from polyethylene terephthalate films (JP-A 313 850/1989) also result on the other hand in an insufficient increase in the adhesion in the case of magnetic recording media having coherent metal layers. Owing to the long exposure times with continuous UV lamps, of the order of magnitude of a few minutes, the process is very time-consuming and does not permit high processing speeds.
It is also known that a periodic dot pattern or ripples can be produced on the surface of stretched PET films by exposure to unpolarized UV light of an excimer laser (E. Arhenholz et al., Appl. Phys. A 53 (1991), 330). These structures occur in an energy range in which the exposure to the laser leads to removal of material (laser ablation). The structures have a typical spacing of a few .mu.m and do not form completely until after exposure of the films to several pulses (at least 3 to 4). The structure spacing increases with increasing energy and number of pulses, as does the height of the structure, which may be several .mu.m. Exposure of the PET materials to polarized UV excimer laser light with about 1,000 pulses and repetition rates of about 3 Hz at anergy densities which are within a narrow range below the ablation threshold and at which the periodic ripples were obtained on the surface is also known (Bolle et al., Appl. Phys. Lett. 60 (1992), 674). Both the height of these structures and the lateral spacing are generally too large for the films treated in this manner to be used as substrates for magnetic thin-layer media.